In U.S. Pat. No. 6,549,120 issued Apr. 15, 2003 to de Buda, “Device for sending and receiving data through power distribution transformers,” which is incorporated herein by reference, the idea of a line-mounted power line monitor which collects data on power flow in an electrical distribution feeder is presented. In order for such a power line monitor to be able to measure energy and power it must be able to accurately measure both voltage and current. There are numerous examples in the state of the art of current sensors which would be suitable; one example being the sensor described in U.S. Pat. No. 6,965,225 issued Nov. 15, 2005 to de Buda, “Coreless current sensor,” which is incorporated herein by reference.
However, development of such a line-mounted power line monitor has been impeded by the lack of a suitable voltage sensor. While there are many examples of voltage sensor technology available, the vast majority of them require connection either to earth-ground or to a known voltage. This requirement presents a serious problem for the line-mounted power line monitor for two reasons. Firstly, distribution feeders operate at high voltages (1,200 to 44,000 volts) and can deliver very high energy levels. Should there be a short circuit in a device that is connected between earth-ground and the feeder conductor; a powerful explosion could easily result, making the installation of any such device very hazardous to utility workers. Secondly, the geometry of the pole configuration may prevent safe access to earth-ground for such a device. An example of this would be the mounting of all three phases of a three-phase feeder on poles without cross-arms, so that the phases are mounted at different vertical levels on the pole.
In U.S. Pat. No. 4,689,752 issued Aug. 25, 1987, which is incorporated herein by reference, Roosevelt A. Fernandes and William R. Smith-Vaniz present a voltage sensor which does not need a connection either to earth-ground or to a reference voltage. This patent describes a line-mounted power line monitor with an energy and power measurement capability using this particular voltage sensor. It is apparent, however, that this device was developed for transmission lines rather than distribution feeders and does not have the ability to minimize interference from adjacent energized conductors. Transmission lines operate at higher voltages (115 kV to 765 kV) and therefore are spaced farther apart (10 to 50 ft). As a result the amount of interference from nearby energized conductors is likely to be smaller than in the case of distribution feeders.
Furthermore, with transmission lines, voltage sensors can be legitimately calibrated in-situ to calibrate out the effect of the other two phases because if one phase is lost, the other two are shut down as well. In distribution feeder lines where the voltage can be as low as 2.4 kV, the spacing might be only two feet, making the interference from adjacent conductors a more serious source of error. It would not be valid to calibrate out this error because in distribution feeder lines if one phase is lost, the other two phases would not necessarily be shut down. This is significant because the loss of one phase would render the in-situ calibration invalid and result in significant error. Thus while the device presented by Roosevelt A. Fernandes and William R. Smith-Vaniz may be suitable for transmission lines, it would not always be suitable for use on distribution feeder lines, which are often close to other energized conductors having other voltage phases as is commonly the case with three-phase feeders, if accuracy is important.
In Japanese patent application no. 2002-131341 published May 9, 2002, Nakai Yuji presents a non-contact voltage sensor consisting of a cylindrical sensing electrode surrounding the conductor whose voltage is to be measured, with a larger shielding electrode surrounding it to reduce interference. This device presents some of the disadvantages referenced above, since the associated electronic instrumentation requires a connection to earth-ground.
In U.S. Pat. No. 7,397,233 issued Jul. 8, 2008, which is incorporated herein by reference, Thomas Sorensen presents a non-contact voltage sensor which has features for reducing external interference. This sensor is based on one set of directional sensors facing toward the conductor whose voltage is to be measured and another set of directional sensors facing away from it. The directionality of these sensors depends on shielding electrodes which are larger than the sensing electrodes. These shielding electrodes are all connected to a common reference conductor which also serves as the voltage reference for the associated electronic instrumentation. One would normally expect the highest level of noise immunity for such a system to be achieved if such a reference conductor were connected to earth-ground, however, Sorenson proposes that the reference conductor be left floating such that it is tied to earth-ground only through stray capacitance. Since such a reference conductor will also have stray capacitance to nearby energized conductors, this creates a potential path for external interference to enter the circuit and produce error. Sorenson indicates for this sensor that the interference pickup is a factor of seven or more less than levels recorded when using the measuring device of his FIG. 2 which is acknowledged to suffer from interference from external fields and is therefore not usable in many applications for accurate power factor and quality measurements. Sorenson also acknowledges that with his sensor it is necessary to know the value of the stray capacitances to the sensor electrodes in order to accurately predict the amplitude of the voltage of the conductor whose voltage is to be measured and instead proposes that the sensor be used for power factor and power quality measurement where the absolute value of the voltage is not necessarily required.
For accurate energy and power measurement the absolute value of the voltage is required and in fact must be determined quite accurately. Thus it is necessary for the voltage sensor to have a very high level of rejection for external interference, without being dependent on stray capacitances which can change due to weather conditions.
There is therefore a need for an accurate voltage sensor which does not need a connection either to earth-ground or to a reference voltage, and which would be suitable for use in a line-mounted power line monitor which must be able to measure energy and power accurately even in the presence of nearby energized conductors.